Remove serial console so all console goes to tty0

While booting, some things require a working console. One of those
things is cloud-init. For whatever reason, ttyS0 being listed as the
last console causes problems with /dev/console usage in Ubuntu 13.04
(raring) that we did not see in 12.10 (quantal).

By using the remove-serial-console element in the seed configuration we
side-step this issue, leading to cloud-init running properly (and thus
generating ssh keys properly).

Fixes bug #1208912

Change-Id: Ic645f58923c869c881ec05b74aa0c822a32e81eb